The process described herein relates to improving the efficiency of an above-ground oil shale retort. More particularly, the process relates to oil shale beneficiation wherein organic-rich oil shale is separated from organic-lean oil shale.
The term "oil shale" as used in the industry refers to a sedimentary formation comprising marlstone deposits with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. The formation containing kerogen is called "oil shale"; herein the liquid hydrocarbonaceous product produced upon decomposition of kerogen is called "shale oil".
The grade of oil shale is commonly stated in units of the quantity of shale oil that can be recovered from the oil shale in a standard retorting test. Most commonly the Fischer Assay is used, wherein a weighed sample of pulverized oil shale is heated in a closed vessel at a specific heating rate. The shale oil distilled is measured and the grade is stated in units such as gallons of shale oil per ton of oil shale. Shale devoid of kerogen produces no shale oil. Kerogen-rich oil shale can produce 70 gallons per ton (gpt) or more.
In above-ground or surface retorts, the retorting process occurs in large metal vessels. Heat supplied to the process may be from direct internal combustion of fuel or from indirect heating of a medium which is then added to the shale. The direct heating method utilizes the combustion of recycled gas or residual carbon in the spent shale with added oxygen or air as the heat source. The indirect heating method uses a furnace or other heating apparatus to heat a solid or gaseous medium which is then introduced into the shale in the retort. Combination processes having both capabilities are also used.
Many methods for shale oil production are described in Synthetic Fuels Data Handbook, Second Edition, compiled by Dr. Thomas A. Hendrickson, published by Cameron Engineers, Inc., Denver, Colo. Examples of the abovementioned processes include the N-T-U and Union Oil direct heating processes, the Tosco II indirect heating process and the Paraho process having both capabilities.
The N-T-U process is a batch process as described at page 67 of the Synthetic Fuels Data Handbook and the U.S. Patents referred to therein. In the N-T-U process, a retort is filled with a batch of oil shale particles and ignited at the top. Combustion is supported by air injection and a combustion zone is passed downwardly through the stationary bed of shale. A recycle of gas from the bottom of the retort is mixed with the combustion gas to modulate temperatures and provide some of the fuel requirement.
The Tosco II retorting process is described at page 85 of the Synthetic Fuels Data Handbook and in U.S. Pat. No. 3,025,223. This process involves preheating oil shale having a particle screen size of 0.5 inch or less to about 500.degree. F. in a fluidized bed. Pyrolysis is completed in a rotating drum heated by ceramic balls which are separately heated in a ball-heating furnace.
The Paraho process is described on page 100 of the Synthetic Fuels Data Handbook. The Paraho process employs a vertical kiln through which ground oil shale moves downwardly as gas moves upwardly. Combustion air can be admitted into the bed of oil shale particles for direct heating of oil shale by combustion within the bed. The kiln can also be arranged so that recycled gas can be heated externally, then injected into the bed of oil shale for indirect heating.
Surface retorting processes have the advantage that process parameters are easily controlled whereas in situ retorting process parameters are more difficult and complicated to control. However, surface retorting processes suffer from the disadvantage that large amounts of shale must be mined, transported to the retort facility and retorted in costly retorts. This adds a tremendous expense to surface retorting operations.
Increasing the efficiency of a surface retorting process is extremely desirable. The cost of producing a specific amount of shale oil in a surface retort is dependent upon the volume of raw material that is required to produce that amount of shale oil, which in turn is dependent upon the concentration of organic material in the raw oil shale. By increasing the concentration of organic material in the surface retort, the cost of producing shale oil is reduced.
Roberts and Schaeffer Resource Service, Inc. (RSRS) have disclosed an oil shale beneficiation process using gravity separation in a heavy medium cyclone to separate oil-rich (i.e., high kerogen content) shale from oil-lean shale. The oil-rich shale is passed to a surface retort wherein shale oil is produced and the oil-lean shale is discarded.
While the RSRS beneficiation process improves the efficiency of a surface retort process, it has poor selectivity in separating organic-rich particles from organic-lean particles. Reduction in the cost of producing shale oil is thereby limited by the poor selectivity of this beneficiation process.